How does DME know when to stop cranking?
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Joined: Aug 2018
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From: Abbotsford, BC
My Ride: 2009 535xi Touring 6MT, 2019 i3s
Engine: N54
How does DME know when to stop cranking?
Just a question out of curiosity since I've not had any trouble yet. When I press the Start button how does the DME know when to stop cranking? I've worked on standby generator control boards in the past and they usually look at engine RPM or generator output voltage to know when it's able to run on its own. My car has always done a simple 1-2 second crank and away it goes. Was curious if I ever did something like draining fuel lines or something and it were to need a few seconds to crank before getting pressure, how it would know how long to engage the starter.
This is on a 2009 535xi, but obviously applies to any BMW with a Start button.
This is on a 2009 535xi, but obviously applies to any BMW with a Start button.
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New Members
Joined: Aug 2018
Posts: 655
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From: Abbotsford, BC
My Ride: 2009 535xi Touring 6MT, 2019 i3s
Engine: N54
Right, I get that it “cranks until it runs”, what I’m asking is how does the dme determine that it’s running? Is it looking at rpm’s meeting a particular threshold? Alternator producing a minimum voltage? Etc.
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Joined: Dec 2012
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From: Montreal Quebec Canada
My Ride: 2005 545i, premium package, 6sp manual, cold package, navigation package.
Model Year: 2005
Engine: N62
Thread Starter
New Members
Joined: Aug 2018
Posts: 655
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From: Abbotsford, BC
My Ride: 2009 535xi Touring 6MT, 2019 i3s
Engine: N54
Of course, it’s really easy for a human to be trained to know when to let go of the key. As a Software Developer I am saying that it can be complicated to tell by sensor data alone since the situation can vary over time. I’ve had engines in the past that sometimes needed to be cranked for quite a few seconds because they were stumbling and not fully catching. That scenario would be difficult for a dme in my opinion. The dme has to weigh whether the engine can run on its own against running the starter motor for too long. I don’t think that’s an easy problem to solve so am trying to learn from others who might know, how the dme comes to the decision that it can disengage the starter.
Remember that the dme can’t “feel” the engine stumbling or hear how the exhaust sounds. It has microphones in the knock sensors but engine won’t be knocking at startup necessarily. It also can tell if a cylinder is misfiring but I don’t know how reliable that is at the low startup speed.
Remember that the dme can’t “feel” the engine stumbling or hear how the exhaust sounds. It has microphones in the knock sensors but engine won’t be knocking at startup necessarily. It also can tell if a cylinder is misfiring but I don’t know how reliable that is at the low startup speed.
Last edited by kd7iwp; Feb 5, 2019 at 06:20 AM.
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From: Chestermere AB
My Ride: 535xi
Model Year: 2008
Pretty simple. Crank Position Sensor. Whilst cranking, the pulses from the CPS are slow. When the engine starts, the pulses speed up and the DME then stops cranking as it now 'knows' the engine is running.
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From: Florida
My Ride: 2008 535xI Wagon
Model Year: 2008
Engine: N54
i would say Tach signal. the RPM's jump once the engine is running.
however, any of the sensors on the engine could be used singularly, or together, to let the DME know when the engine is running.
however, any of the sensors on the engine could be used singularly, or together, to let the DME know when the engine is running.
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Joined: Aug 2011
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From: Baton Rouge, LA
My Ride: 2008 550i
Model Year: 2008
Similar to what audiophool said. Not sure at what speed the engine cranks, but each crank only lasts a split second.
So I would assume the DME needs to see constant engine revolutions over a given time (say 1-2 seconds) AND engine revolutions at or above idle speed of 500rpm.
This data would signal to the DME that the engine is now running on internal combustion and not the starter motor anymore.
So I would assume the DME needs to see constant engine revolutions over a given time (say 1-2 seconds) AND engine revolutions at or above idle speed of 500rpm.
This data would signal to the DME that the engine is now running on internal combustion and not the starter motor anymore.
Last edited by dingolfing; Feb 5, 2019 at 08:44 AM.
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Joined: Oct 2012
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From: Chestermere AB
My Ride: 535xi
Model Year: 2008
Cranking speed is much lower than idle speed. Pulses from the CPS (it reads a toothed wheel using Hall Effect (with signal conditioning)). Say the pulses give a 500Hz signal whilst cranking, the signal jumps to 1500Hz once the engine starts running. The DME reads the higher frequency pulses and says "Hey! I'm running now, turn off the starter!"
